I. Field of the Invention
The invention relates generally to automatically controlling the speed of a moving object and, more particularly, to automatically adjusting the speed loop gain of a feedback control system used in, for example, a numerical control system controlling a machine tool, a robot, or the like.
II. Description of Related Art
In a controlled machine tool, various parameters affecting the characteristics of a feedback control system must be set by a manual input operation for the final optimization adjustment of the feedback control system. Generally, a feedback control system for systems equipped with servomotors, such as a machine tool, robot, etc., is divided into a position control loop, a speed control loop, and a current control loop. The speed control loop generates a speed command corresponding to the difference between a speed command from the position control loop and an actual motor speed. The speed control loop has a response characteristic that is determined in dependence on the speed loop gain. That is, when the speed loop gain is excessively small, the motor rotation will not immediately follow each movement command pulse supplied from the servo system and a control lag occurs in such a way that the motor cannot start rotating before a plurality of pulses are accumulated, thus causing a low-frequency undulation in the motor speed. Further, the servo system is liable to be effected by disturbance. On the other hand, when the speed loop gain is excessively large, the control stability is degraded.
A conventional optimization adjustment procedure for the feedback control system requires operators to examine the accuracy of motions of the machine tool. During optimization, the parameters are determined by a trial and error method while examining the accuracy of motions by using measuring instruments. The adjustment operation of a feedback control system requires operators with considerable experience and high skill to keep the feedback control system in optimum condition, because important characteristics of the feedback control system conflict with each other. Stability is deteriorated if accuracy and response characteristics are enhanced and vice versa. Currently, numerically controlled machine tools and industrial robots require a feedback control system having highly improved performance, resulting in more operations for optimization that become time consuming and complicated.
Accordingly, there is a need for an automatic speed loop gain adjustment method and apparatus for a feedback control system.
The present invention provides an automatic speed loop gain method and apparatus for a feedback control system. We automatically adjust the speed loop gain of a feedback control system by setting up a predetermined phase margin range and a predetermined gain margin range. We provide a speed open loop condition by separating a value of speed command from a speed feedback value. White noise is input into the feedback control system where the speed open loop condition has been made in a cycle that is substantially the same as a control cycle of the feedback control system. We detect a phase margin and a gain margin at the time the white noise is input into the feedback control system. Then, we compare the detected phase margin and the detected gain margin with the predetermined phase margin range and the predetermined gain margin range, respectively. We then adjust the speed loop gain if the comparison results for either one of the detected phase margin and the detected gain margin fall outside of the associated predetermined phase margin range or predetermined gain margin range. Adjusting the speed loop gain is effected by increasing the speed loop gain in a stepping state according to a predetermined unit quantity that is set up as a parameter beforehand, wherein the speed loop gain does not exceed a predetermined speed loop gain range. We stop adjusting the speed loop gain if the comparison results of both the detected phase margin and the detected gain margin fall within the associated predetermined phase margin range and predetermined gain margin range. Additionally, we can adjust the position loop gain of the position control loop once the adjustment of the speed loop gain is complete, wherein the position loop gain is adjusted on the basis of the adjusted speed loop gain.
An automatic speed loop gain adjustment arrangement for carrying out the above described method includes a position control unit to receive a position command value and a feedback position value to produce and output a speed command value based on a deviation between the received position command value and the feedback position value, a speed control unit to receive a speed command and a speed feedback value to produce and output a current command based on a deviation between the received value of the speed command and the feedback speed value, and a current control means receiving a current command and a feedback current value to produce and output a current command value based on a deviation between the received value of the current command and the feedback current value. We also provide the automatic speed loop gain arrangement with a white noise input element between the position control unit and the speed control unit to inhibit the transfer of the speed command to the speed command unit upon reception of white noise. In addition, we provide gain margin and phase margin determination elements to determine a phase margin and a gain margin upon reception of white noise by the white noise input element, a comparison element to compare the determined phase margin with a predetermined phase margin range and the determined gain margin with a predetermined gain margin range, and speed loop gain adjustment element to adjust a speed loop gain if the comparison element determines if either one of the determined phase margin and the determined gain margin falls outside of the associated predetermined phase margin range or predetermined gain margin range. The automatic speed loop adjustment arrangement may also include a position loop gain adjustment element to adjust the position loop gain on the basis of the adjusted speed loop gain.